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<br />at the crest ot a recent stre&lll nse had been tOWld to have been J.OO:JY <br />c.t.s. What conclusions could the highway engineer have drawn trom <br />such a measurement1 ~ that 1000 c.t.s. could be counted on as be1na <br />one ot the almost 1nt1n1 te lUllllber ot peak rates that might be expected <br />to occur in the tuture. It a second measurement had been ma.de three <br />months later at the crest ot another strelllD nse 1 t migtl't have been <br />tound that the peak rate tor that rise was 10,000 c.t.s. or 10 t1mes <br />the peak rate at the time ot the first measurement. It a culvert had <br />been built Oil the basis ot a 1IIP1'lP'lII peak rate of 1000 c.f.s. it is <br />probable that it wuld have been washed out 1>7 the 10,000 c.t.s. flood. <br /> <br />Frequency ot Occurrence ot Indi vidnal Peaks <br /> <br />Should. the above culvert bave been designed. for 10,000 c.t.s., <br />15,000 c.t.s. or for some higher rate1 It would be helpf'Ul ill answering <br />this question if we knew how trequently we could expect stoms that <br />producei peak =ff rates ot 10,000 c.f.s. Was the 10,000 c.t.s. peak <br />a rnre occurrence or could we expect it to repeat itself every three or <br />four years1 <br /> <br />To detenoine how frequently we might expect peak rates ot various <br />l:l8.gIlitudes, wouJ.d require a record ot the past history ot the strelllD. <br />.e could then make the asSUlllption that the frequencies observed. 1n this <br />record would be repeated in the tutl.lre. '!'he U. S. Geological Survey <br />provides us with just such a history tor thouS8D.ds ot strelllDB. The <br />da"ta derived. from their meas\U'elllents ~ publlshed in a series ot <br />,iater Supply Papers that include the 9maximum peak rate ot runoff <br />experienced. during each ot the years ot record.. <br /> <br />Let us su.ppose that our proposed. strelllD crossing was part ot the <br />Geological SUrvey's hydrologiC network and that strelllD flow measurements <br />were available tor the past y) years. An eY"oMf\8.t10n ot the first 10 <br />years of record. might have shown that the highellt ma"1...... lIDIII&&l peak rate <br />ot =tt tor the period occurred. during the fif'th year and vas equal to <br />12,000 c.f.s. It this were all ot the record available, we m1ght have <br />concluded that every ten years we could expect a peak rate ot 12,000 c.t.s. <br />An examination ot the balance ot the record would probably have shown <br />us how wrong lIu.ch a conclullion would have been. The next 10 years ot <br />record. m1l5ht have shown that the h1ghest IIIIIX1muIII annual peak rate tor <br />this period was o~ 5,000 e.t.s. and that the IIIBX1mum .nn...' peak tor <br /> <br />y e.t.s. = cubic teet per second <br /> <br />y 'l'h1s IIlIlv1...... peak rate ot :runoff in ~ year 1s <br />hereafter as the mav1...... annual peak rate tor that year. <br /> <br />reterred to <br /> <br />3-2 <br /> <br />e <br /> <br />" <br /> <br />~. <br /> <br />e <br /> <br />". <br /> <br />) <br /> <br />e <br />